1. Field of the Invention
The present invention relates to α-sialon phosphor that is excited by ultraviolet rays or blue light to emit orange or red light, method for producing same, and a light-emitting apparatus using the α-sialon phosphor.
2. Description of the Related Art
As nitride or oxynitride phosphor, α-sialon phosphor has been known to be superior not only in the fluorescent emission efficiency but also in the temperature characteristic. In particular, europium doped α-sialon phosphor is excited in a wide wavelength region from ultraviolet rays to blue light to emit yellow to orange light. Thus, this α-sialon phosphor has been examined to be used as a yellow phosphor as a substitute for YAG:Ce or to be used as an incandescent color Light Emitting Diode (LED) phosphor that has a low color temperature (Patent Literatures 1 and 2, Non-Patent literature 1).
The α-sialon is structured so that the Si—N bond of an α-silicon nitride crystal is partially substituted with an Al—N bond and an Al—O bond and, in order to maintain the electroneutrality, specific elements (Ca and Li, Mg, and Y, or lanthanide elements except for La and Ce) are penetrated and solid-solute to the crystal lattice. A part of the elements thus ingressed and solid-soluted functions as a light emission center element, thereby providing a fluorescent characteristic. The α-sialon can be synthesized, for example, by subjecting, to a heat treatment in a nitrogen atmosphere, mixed powders of silicon nitride, aluminum nitride, and the oxides of the ingressed and solid-soluted elements including a light emission center. Oxides of the ingressed and solid-soluted elements also include compounds that turn into oxides by a heat treatment. In the case of this synthesization method, oxides are used as a raw material, thus inevitably causing the synthesization of α-sialon in which a certain amount of oxygen is ingressed and solid-soluted. In the case of a α-sialon phosphor having a superior fluorescent characteristic in which Ca and Eu as a light emission center are ingressed and solid-soluted, α-sialon in which a certain amount of oxygen is ingressed and solid-soluted emits yellow to orange light (having a fluorescent peak wavelength of about 580 nm).
On the other hand, α-sialon having low oxygen content rate synthesized by using calcium nitride as a calcium raw material can have a calcium solid solution ingressed therein to have a higher concentration than in the case of the known α-sialon as described above. It has been known that, if the Ca solute concentration is high in particular, such a phosphor is obtained that has a fluorescent peak wavelength at the higher wavelength-side (595 nm or more) than in the case of the conventional composition using an oxide raw material (Patent Literature 3). In addition to α-sialon, a nitride phosphor having low oxygen content has been reported (Patent Literature 4).